1. Field of the Invention
The present invention relates to a memory system including a nonvolatile semiconductor memory.
2. Description of the Related Art
In recent years, nonvolatile semiconductor memories such as a flash memory for storing information according to an amount of stored charges are widely known. Recently, a capacity of a NAND flash memory is increased. A personal computer incorporating the NAND flash memory as a secondary storage device is put to practical use.
The NAND flash memory is a semiconductor memory in which erase processing is necessary before writing data. The durable life of the NAND flash memory depends upon the number of times of erasing. Data erasing and data writing for the NAND flash memory are performed by applying high voltage between a substrate and a control gate to inject electrons into and discharge electrons from a floating gate. Therefore, when erase processing and write processing are performed a large number of times, a gate oxide film around the floating gate is deteriorated and the electrons injected into the floating gate are lost. It is likely that recorded data is broken.
Data recorded by a computer such as a personal computer has temporal locality and regional locality (see, for example, David A. Patterson and John L. Hennessy, “Computer Organization and Design: The Hardware/Software Interface”, Morgan Kaufman Pub, 2004 Aug. 31). Because of this characteristic, when data is recorded, if the data is directly recorded according to an address designated from the outside, rewrite processing and erase processing are concentrated in a specific area and respective storage devices are unequally deteriorated. Therefore, in the NAND flash memory of this type, processing called wear leveling for equally distributing data update sections in a semiconductor memory is performed to generally equalize the number of times of erasing in all memory cells in the semiconductor memory.
As explained above, in the secondary storage device including the nonvolatile storage memory such as the NAND flash memory, it is necessary to sufficiently taken into account management of the memory cells compared with a secondary storage device including a hard disk. Various technologies not adopted in the secondary storage device including the hard disk are necessary.